A question about how AAD adjust their ground reference (all brands, really, but I have a Cypress):

The Farm DZ (LZ, manifest, hanger, packing area, beer refrig, etc) is located about 130 ft below the airport runway elevation. (IOW, the unit is turned on, and almost all non-jumping time is spent, at the DZ). The drive over is about 4 miles, pretty much flat until we ascend a hill (over the course of 60 sec or so) and arrive at the airport. Once there, time on the ground before lift off might range from 5 to 10 minutes.

Would a Cypres reset it's ground reference elevation to that of the airport?

How about other AAD brands?

(Yes, I did RTFM, but it only spoke generally about making adjustments during the day to account for weather changes. I'm hoping someone might have a bit more specific knowledge about how it adjusts and whether the time spend at the airport would cause it to adjust it's ground reference upward.)

They set as of the location that they are turned on. For a dropzone with a small elevation difference between the airport and landing area it's not a huge deal. For other dropzones (Hollister, CA is one example - you do a 450 foot offset, though it's been so long since I jumped there that I can't remember which direction it goes) where the difference between locations is greater you need to do an offset.

This incident shows what can happen if you turn your AAD on at a significantly different altitude than that of the dropzone.

But the issue is this: the Cypres does adjust it's ground reference pressure to account for changes in atmospheric pressure (so that, for example, if a high pressure front moved it and changed local barometric pressure at the DZ, it will adjust to that change). So my question is whether or not the 5-10 (or 15, if delayed) min at the airport 130 higher would result in the AAD instead thinking that the local barometric pressure had changed (rather than the altitude).

I put the question to Airtec (mfg of the Cypres) and they indicated that my Cypres likely would adjust its ground reference to that of the airport under the scenario I gave. In effect, then, my Cypres effectively has a ~130ft offset (~40 meters) upward anytime we depart from the airport. (I.e., it's trigger altitude would effectively be 750+130 = 870 ft above the LZ.)

For me, I actually would prefer this, as there are some hills near the Farm (completely forested!) that are ~100 to 140 ft higher than the LZ. So given that the offset 1) is not very big and 2) it is in an arguably beneficial direction (particularly with respect to the surrounding terrain), it is something I am comfortable living with.

Presumably other brands of AAD would do a similar adjustment, although it would depend on their algorithm for tracking ambient pressure.

@sparky As I said, I did RTFM and I do understand how one would do that. But that was not my question. Rather, my question was an attempt to find out what the Cypres would do in that situation. Would it adjust to the airport level? Or, given the amount of time spent at the airport, would it keep its ground reference at DZ level? (Or would it be somewhere in between?) That specific information is not in the manual, and would depend a lot on the specific algorithm used to track changes in ambient atmospheric pressure relative to the algorithm used to detect airplane takeoff.

@piisfish They recommended that I set an offset at the airport if I wanted the trigger level to be based on DZ altitude. They also said that if I didn't mind a ~130 ft offset, then I could just do nothing. I'll note that I have never observed another jumper cycling their AAD and setting an offset at the airport, so I'd wager that virtually every Farm jumper effectively has a ~130 ft offset on their AAD for loads originating at the airport.

[reply I'll note that I have never observed another jumper cycling their AAD and setting an offset at the airport, so I'd wager that virtually every Farm jumper effectively has a ~130 ft offset on their AAD for loads originating at the airport.do they at least restart their Cypreses on arrival at the airport ? if not, that would be plain WRONG (and they didn't read/understand the manual) Same with the Vigils, but these should be turned off prior to boarding the vehicle

The manual does say "When the takeoff airfield and intended dropzone are in different locations, CYPRES must be switched on at the departure airfield."

But it does not say WHY. Will the Cypres catch fire? I don't think so. Will the calibration be either that of the DZ where the Cypres was turned on, or possibly the airfield one drove to, depending on the time and rate of altitude change involved? Yes, sounds like one of these will be the case, and with the airfield higher than the DZ, 130' may be an acceptable offset.

So the interpretation of "you must do something" depends on what your goal is, and what the consequences are.

Divalent has contacted Airtec who seem to have said that he could just leave the Cypres on. So while it is good to have a careful reading of the manual, in this case the information from the company EXPANDS ON what the manual says, which also means that it CONTRADICTS what is in the manual.

This is an interesting case where the manual is right in a sense but does not cover all possible reasonable courses of action. The manual does give a method that takes a little more time but makes it unambiguous what altitude the AAD is set to.

Divalent sees everyone else just leaving their AAD on and happens to go a little further in investigating the situation, so he's the one dealing with the arguments...

Well, I think pchapman responded on point to this. I'll just note that the general recommendation to recycle and put in an offset at the take-off point whenever the elevations differ has to have some practical limits which take into account the magnitude and direction of the difference, the reasons for doing it, and the effort to do so.

A 20 ft difference either direction I don't think anyone would worry about. A 130 ft higher airport (as we have at the Farm) looks like it gives you two acceptable choices: you can either recycle and offset, or accept that the trigger altitude is raised by ~130 ft. But 130 ft in the lower direction, you might want to recycle and offset (unless your are comfortable with a 620 ft trigger altitude).

>The manual does say "When the takeoff airfield and intended dropzone are in >different locations, CYPRES must be switched on at the departure airfield."

We called Airtec specifically on this when we jumped at Brown (road between DZ and airport about 150 feet higher) and they said we did not need to cycle AAD's in that case. It's worth a call to Airtec to see what they say in this case,